Add context-aware "time" library wrapper.

go/src/infra/libs/clock/systemtimer.go

Index: go/src/infra/libs/clock/systemtimer.go
diff --git a/go/src/infra/libs/clock/systemtimer.go b/go/src/infra/libs/clock/systemtimer.go
new file mode 100644
index 0000000000000000000000000000000000000000..84b31dc5e8eba1efcff76df918d2147c0477ada0
--- /dev/null
+++ b/go/src/infra/libs/clock/systemtimer.go
@@ -0,0 +1,38 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package clock
+
+import (
+	"time"
+)
+
+// A Timer implementation that uses time.Timer.
+type systemTimer struct {
+	T *time.Timer // The underlying timer. Starts as nil, is initialized on Reset.
+}
+
+var _ Timer = (*systemTimer)(nil)
+
+func (t *systemTimer) GetC() (c <-chan time.Time) {
+	if t.T != nil {
+		c = t.T.C
+	}
+	return
+}
+
+func (t *systemTimer) Reset(d time.Duration) bool {
+	if t.T == nil {
+		t.T = time.NewTimer(d)
+		return false
+	}
+	return t.T.Reset(d)
+}
+
+func (t *systemTimer) Stop() bool {
+	if t.T == nil {
+		return false
+	}
+	return t.T.Stop()
+}